The teachings herein relate generally to techniques for rapid compensation of phase and amplitude information in an electrical signal.
Many countries now require that wind turbines used as electric generation facilities stay connected with the electric grid when the grid is in fault. Remaining connected during system fault (often referred to as “low voltage ride through”) can be challenging from an engineering perspective. Perhaps most importantly, the phase and amplitude information of the sequence components in the grid signal must be made available to the turbine control systems quickly and accurately. This permits the control systems to make timely adjustments to the wind turbine, thereby mitigating the effect of any large signal transients and thus prevent tripping of the wind turbine.
Generally, the primary goal of a power generation asset is to control positive sequence voltage. Traditional current regulated approaches implicitly attempt to eliminate the negative sequence current. For a wind turbine system using a Doubly Fed Induction Generator, in the presence of a load fault or imbalance condition, this requires a rotor side converter to support a negative sequence voltage and supply negative sequence current. Unfortunately, turbine systems may be limited in their ability to supply adequate negative sequence voltage, current or power. This leads to degradation of system controllability, and repeated operation of protection measures (e.g., a “crowbar circuit”), thus subjecting the generator and other turbine components to repeated transients. Dynamic brake resistors may be applied to shunt power from the DC bus, limiting activation of the crowbar and maintaining controllability.
Protection of a branch or feeder circuit coupled to the grid may depend upon the circuit having a low impedance characteristic for negative sequence voltages. That is, it may be expected that the branch circuit is capable of supplying some current to a negative sequence fault or imbalance condition. Typically, this depends upon the capacity of the grid to support the fault condition. Further, with various performance standards for generation assets, specifications for equipment in some instances may require negative sequence current in response to negative sequence voltage. Unfortunately, traditional current management schemes typically impede techniques for coordinated protection of branch and feeder circuits.
Traditional solutions to address these challenges have resulted in systems with non-linear behaviors, making it difficult to provide simple models of sub-system elements for use in full-system models. Design of experiments typically becomes quite complex, as exhaustive scenarios are needed to attempt to cover the locus of the non-linear system.
A number of resources have been directed to addressing or examining grid disturbance operation for generation assets. Examples include a technique described in the paper “Vestas Handles Grid Requirements” Advanced Control Strategy for Wind Turbines,” by Bolik, et al. wherein numerous steps are taken, the first of which is disconnect the stator of the generator from the grid.
A second paper “Transient Analysis of Doubly Fed Wind Power Induction Generator Using Coupled Field-Circuit Model, by Seman et al. has examined aspects of grid faults. In the approach disclosed by Seman, the rotor side frequency converter is controlled by a modified direct torque control (DTC) control strategy.
A third paper “Comparison of Fault Ride—Through Strategies for Wind Turbines with DFIM Generators,” by Dittrich, et al., compares various fault ride-through strategies.
A fourth paper “Experiences on Voltage Dip Ride Through Factory Testing of Synchronous and Doubly Fed Generator Drives,” by Niiranen discloses techniques for fault emulation and measuring aspects related to system faults.
A number of the prior art techniques for responding to grid disturbances call for application of a crowbar circuit. Typically, when using this approach, the power generation equipment is unable to respond properly in light of new standards and demands for generation systems.
What is needed is a technique for maintaining a generation asset, such as a wind turbine, coupled to an electric grid during low voltage periods or periods of grid signal instability, wherein the technique coordinates the voltage, current and power capabilities in such a way as to account for positive and negative sequence grid conditions to extend small signal linearity and provide for reductions in self-protection transients.